1. Field of the Invention
The field of the invention is that of the broadcasting of digital signals in channels allocated multiple to paths, the characteristics of which vary in time.
The invention concerns notably the broadcasting of digital data intended to be received notably by mobile receivers in an urban environment, that is, in the presence of parasites and jamming and under conditions of multiple propagation (the RAYLEIGH law or process) generating phenomena of fading.
The invention can be applied more particularly, but not exclusively, to the system of digital sound broadcasting known as the COFDM (Coding Orthogonal Frequency Division Multiplex) as described in the French patent applications Nos. 86 09622 of Jul. 2, 1986 and 86 13271 of Sep. 23, 1986 filed on behalf of the present applicants.
This system of digital broadcasting is based on the combined use of a channel coding device and a method of modulation by orthogonal frequency division multiplexing.
2. Description of the Prior Art
The known encoding method is aimed, notably, at absorbing the effect of variations in amplitude of the received signal, due to the RAYLEIGH process, by an adequate processing of the samples coming from the demodulator. Preferably, a convolutional encoding is used, possibly concatenated with a REED-SOLOMON type of encoding.
The decoding may advantageously be a soft decision type of Viterbi decoding.
The modulation method proper to this prior art system makes it possible to resolve the problems related to the frequency selectivity of the channel. It consists in providing for the distribution of the constituent digital elements of the data signal in the frequency time space f-t, and in simultaneously transmitting sets of digital elements on N parallel broadcasting channels by means of a multiplex of frequencies using orthogonal carriers.
In a known way, the encoded digital elements are furthermore interlaced, in time and in frequency, so as to maximize the statisical independence of the channels with regard to the Rayleigh process and the selective character of the channel.
In particular, this makes it possible to prevent two successive elements of the data train from being transmitted at the same frequency.
The two-dimensional nature of the transmitted signal necessitates the implementation, at the receiver, of a sampling along the two axes, namely the temporal and frequency axes. The determining of the sampling instants in time is the result of the temporal synchronization of the receiver. The frequency sampling arises out of the application of a discrete Fourier transform (DFT) on the temporal samples.
This transform is equivalent to a bank of orthogonal filters, the position of which, in the frequency domain, is determined by the frequency of the local oscillator. The precision of the sampling in frequency depends, therefore, on the frequency distance between the transmitted signal and this local oscillator.
The known operation, which consists in tuning the frequency of the local oscillator, in the receiver is called automatic frequency control (AFC). A method of application of AFC is described, for example, in M. Alard, R. Halbert, B. Le Floch, D. Pommier, "A New System Of Sound Broadcasting To Mobile Receivers", Eurocon Conference 88.
A frequency shift of the bank of filters, achieved by the Fourier transform, has two distinct effects on the demodulation of a particular carrier of a COFDM signal. Firstly, the distance between the frequency of the transmitted signal and the frequency of the local oscillator introduces a phase error. Furthermore, after Fourier transform, this distance introduces a noise due to the loss of orthogonality between the carrier frequencies, expressing the inter-symbol interference in the frequency domain.
Depending on the precision of the oscillators, it may be that the AFC is not sufficient to provide for efficient locking-in. Thus, in the case of a COFDM system where the distance between carriers is equal to 10 kHz and the carrier frequency is 1 GHz, a precision of 10.sup.-5 of the local oscillator may prompt a shift of a carrier in the demodulator, making any interpretation of the received signal impossible.
The invention is designed to overcome these drawbacks.